A Study of In Situ Combustion in a Segregated System

Khelil, Chakib, Member AIME, D.R. McCord and Associates, Inc. Some or the field pilot studies made to date have indicated that forward combustion proceeds in a segregated manner in oil proceeds in a segregated manner in oil reservoirs and that surfaces at which combustion is occurring develop both parallel and perpendicular to the general direction of air injection. Although mathematical studies or combustion in a segregated system have been made, no experiments involving such a system have ever been attempted. The objectives of this investigation are to study experimentally the factors affecting oxygen consumption and oil recovery in a segregated system and to provide a better mathematical means than those now available for estimating the distribution of temperature, oxygen and fuel throughout the reservoir. The mathematic model is also useful in studying the effects of some factors not examined experimentally. Nine combustion tube runs were made wherein all of the factors involved in the process were studied. The objectives of this study were then fulfilled by correlating the data obtained from material balance by means of a multiple regression analysis program. The results obtained showed that burning in a segregated system is characterized by the fingering of the coke zone, the formation of secondary surfaces of combustion, low pressure gradients, and migration of oil into the gas cap. There is more even oxygen consumption at low than at high flux and pressure. Oxygen consumption increases as flux, pressure, clay content, water saturation and oil gravity increases. The recovery of initial oil in place also increases as crude oil gravity place also increases as crude oil gravity increases. It is also noted that recoveries in this type of system are substantially the same as those in similar vertical tube runs but differ in the time of oil breakthrough. The mathematical model has been developed to simulate burning in a segregated reservoir. The models presented by previous authors are shown to represent particular cases of this model. The mathematical study shows that a more rapid burning in the horizontal direction is caused by one or several of the three following factors:heat losses to the surroundings from the oil zone,an increase in gas cap thickness, anda decrease in air flow rate through the oil zone. It is also found that a decrease in gas cap porosity leads to lower heat transport through the gas cap and consequently to slower burning in the horizontal direction.